1. Field of the Invention
The present invention relates to a signal processing method and a signal processing apparatus, and more specifically to a method and apparatus for transforming N-bit codes such as audio signals, video signals, etc. into M-bit codes on the condition that M is larger than N.
2. Description of the Prior Art
In the case where signals such as audio or video signals are converted into digital signals, digital signals having a predetermined number of bits are generated per sample in conformity with a standard prescribed under due consideration of various conditions (e.g., transmission, fidelity of recording and reproduction, apparatus cost, etc.). For instance, in the case of a compact disk, 16-bit digital signals are recorded per sample.
In FIG. 1, a plurality of thick solid lines S as a.fwdarw.b.fwdarw.c.fwdarw.d.fwdarw. . . . k.fwdarw.1.fwdarw.m.fwdarw.n, represent digital signals obtained by quantizing an original analog signal (by resolution of 1/2.sup.N for each specific sampling period Ts) in a form of analog signal. Here, the original analog signal resides in the ranges enclosed by dashed lines including the solid lines S shown in FIG. 1. In other words, there exists an error of less than .+-.0.5 LSB (the least significant bit) between the original analog signal and the restored analog signal (obtained by restoring the original analog signal). Therefore, in the N-bit digital signal obtained by converting the analog signal in the resolution of 1/2.sup.N, the minute signal as fine as is obtained by a resolution to more than 1/2.sup.N cannot be restored. Further, in FIG. 1, t1, t2, t3, . . . denote sequential sampling points, and Ts denotes a sampling period.
However, there exists so far a need to restore the minute signal using a resolution of more than a value determined by the number of bits of digital signals. Therefore, for instance, Japanese Published Unexamined (Kokai) Patent Application No. 5-304474 has proposed such a method of transforming N-bit codes into M-bit codes on the condition that M is larger than N. In a technique of increasing the number of bits disclosed in this prior art patent, digital signals are smoothed through a digital low-pass filter so that even the minute level signals can be DA (Digital-to-Analog) converted without distortion. In other words, data less than one LSB of the original number of bits can be outputted for DA conversion.
In this prior art technique, although the N-bit digital signals are converted into M-bit digital signals (M&gt;N) by use of the digital low-pass filter, it is impossible to correct the errors of 0.5 LSB involved in the N-bit digital signals. In addition, since the digital low-pass filter is used for waveform smoothing, the signal waveform changes. As a result, when this method is applied to the DA conversion of audio digital signals, for instance, there exists a problem in that the quality of the audio digital signals varies.